That's a plausible explanation, and it would also explain why the information was not in the Kyodo story, since it would be admitting what we all know, that to a Japanese, one is either 100% Japanese and non-burakumin, or an outsider.
I read a US Navy Intelligence report from 1942, arguing against locking up Japanese Nationals, and US Citizens of Japanese Ancestry. It admitted that there were some immigrants still loyal to Japan, but asserted that the vast majority were not, and those had given the FBI ample in formation about those who were not to be trusted.
One comment that struck me was that the most absolutely, fervently, loyal to the US subpart of that population was those who had been sent to Japan to complete their education. To a man and woman they found that the undoubted discrimination they had experienced in the US, for being of Japanese extraction, was dwarfed to insignificance by the discrimination they had experienced in Japan for not being 100% Japanese. Apparently the Japanese understood that Gaijin could not help being Gaijin, but they had a special loathing for people with Japanese names and faces who were not 100% Japanese.